I recently acquired a HP 419A in reasonable good nick (neons work perfectly, NiCd batteries still hold charge, Hg battery for bucket voltage is dead, glass of MCM lens is fractured), but if I'm not mistaken, the input bias current is more than 2nA (indicated voltage ~220uV across a 100kOhm R). Afaict, the input bias current isn't specified for the HP 419A, but 2nA seems excessive for a null voltmeter, particularly if it is designed to also measure currents in the 30pA range. At first glance it looks quite clean and pretty (gold traces!) inside (and any contamination shouldn't cause a current that large). I need of course a new battery for the bucket voltage generator, but since the bucket voltage spans only 120% of the measuring range, it won't fix the issue. What else could it be?
[shoot -- just realized, this should have gone into the 'repair' section]